1. Field of the Invention
The present invention relates to a voltage-controlled oscillator, a composite module, and a communication apparatus.
2. Description of the Related Art
FIG. 5 is a circuit diagram of a conventional voltage-controlled oscillator 1. The voltage-controlled oscillator 1 includes a resonant circuit 2 and an amplifying circuit 3. The resonant frequency of the resonant circuit 2 changes according to a control voltage Vc applied to a control terminal 4. The amplifying circuit 3 oscillates a signal based on the resonant frequency of the resonant circuit 2 and amplifies the signal.
Referring to FIGS. 6 and 7, the circuits 2 and 3 of the voltage-controlled oscillator 1 typically include various electronic components (described below) mounted on or formed inside a circuit substrate 10, which is produced by integrally firing multilayered ceramic sheets having circuit patterns or ground electrodes printed thereon. In FIGS. 6 and 7, the circuit substrate 10 includes a control terminal 4, a driving power terminal 5, an output terminal 6, inter-layer connecting terminals 7 to 9, and a ground terminal G. The circuit substrate 10 further includes wiring patterns 12a to 12e, a via hole 11a, and a through-hole 11b. In addition to the components described above, the circuit substrate 10 has on its upper surface other wiring patterns, via holes, and through-holes, which are not shown in FIGS. 6 and 7.
The resonant circuit 2 includes a choke coil L1, a variable-capacitance diode D1, coupling capacitors C9 and C10, a resonant capacitor C11, a resonator (strip line or microstrip line) SL2 which is an inductor element, and a bypass capacitor C12.
The amplifying circuit 3 includes a buffer amplifying transistor Tr1, an oscillating transistor Tr2, bias resistors R1, R2, R3, and R4, Colpitts capacitors C4, C5, C6, and C7, bypass capacitors C1 and C3, a coupling capacitor C2, a choke coil (strip line or microstrip line) SL1, a resonant capacitor C8, and a resonant inductor (strip line or microstrip line) SL3 which is an inductor element.
Japanese Unexamined Patent Application Publication No. 11-74727 discloses another well-known voltage-controlled oscillator, which has a different circuit from that according to preferred embodiments of the present invention. In this voltage-controlled oscillator, the emitter of a transistor is connected to the ground not only via a capacitor and a bias resistor, but also via a resonator connected to a node between the bias resistor and the capacitor.
Referring again to FIG. 5, the bias resistor R3 in the amplifying circuit 3 has one end electrically connected to the base of the transistor Tr2 and the other end directly connected to the ground. One approach for connecting the bias resistor R3 to the ground is via the wiring pattern 12a that is provided on the circuit substrate 10, as shown in FIG. 6, so as to electrically connect the bias resistor R3 to the ground terminal G.
This approach, however, prevents chip components from being mounted in an area S1 because the area S1 overlaps the wiring pattern 12a. Therefore, this approach is not appropriate for compact design of the voltage-controlled oscillator 1. Another disadvantage of this approach is that the space available for a microstrip line is reduced which makes it difficult to form the resonator SL2 as a small-loss microstrip line.
Another approach, as shown in FIG. 7, is to form the through-hole 11b in the circuit substrate 10 such that the through-hole 11b electrically connects the bias resistor R3 to a ground electrode (not shown in the figures) on the lower surface of the circuit substrate 10. This approach eliminates the need for forming the wiring pattern 12a on the circuit substrate 10, and thus enables a chip component such as the bias resistor R1 to be mounted in the area S1.
Unfortunately, this approach has a disadvantage in that the area for the resonator (strip line or microstrip line) SL2 inside the circuit substrate 10 is restricted by the through-hole 11b which prevents an area S2 from being utilized, as shown in FIG. 8. Thus, this approach is also not appropriate for compact design of the voltage-controlled oscillator 1. In particular, the resonator SL2 which is defined by a strip line requires the formation of the via hole 11a for electrically connecting between strip lines on the surfaces and the inner layers of the circuit substrate 10, which further restricts the component layout and internal layer patterns.